1. Field of the Invention
The present invention relates to a liquid crystal display device for use with an active-matrix-type liquid crystal display device and an inspecting method thereof, in particular, an inspecting method for inspecting defective pixels on a substrate.
2. Description of the Related Art
In an active-matrix-type liquid crystal display device, switching thin film transistors (TFT) and transparent electrodes are disposed at intersections of data signal lines and gate signal lines so as to control voltages of the transparent electrodes. For example, Si-type liquid crystal display panels that are small and have high resolutions are being increasingly used for cellular phone units, personal digital assistants (PDA), and so forth.
A Si-type liquid crystal display panel is structured in such a manner that liquid crystal is sealed between a large scale integrated circuit (LSI), on which a transistor, a capacitor, and a pixel electrode (for example, a reflection plate) are formed for each pixel on an Si wafer and transparent electrodes coated on a glass substrate. The LSI is produced by, for example, the complementary metal oxide semiconductor (CMOS) process. In the specification, LSI on which reflection electrodes have not yet been formed or in which liquid crystal has not yet been sealed is referred to as a liquid crystal display device substrate.
Generally, since the area of pixel portions of an active matrix liquid crystal display device substrate is large, the non-defect rate of the pixel portions is lower than that of a driving circuit portion. Thus, the production cost of the substrate adversely becomes high. As a result, it is important to improve the non-defect rate of the pixel portions. To improve the non-defect rate, it is essential to develop a method for inspecting defective pixels. As a method for inspecting defective pixels, after filling them with liquid crystal, the liquid crystal is driven and the display image is analyzed by an image analyzing unit. As another method, defective pixels are visually inspected.
However, in such methods, since a liquid crystal display device is actually driven and an image is displayed thereon, defective pixels are inspected. Thus, a long measuring time is required and high productivity cannot be expected. In addition, when defective pixels are inspected after filling them with liquid crystal, even if defective pixels are detected, the liquid crystal display device should be disposed. This is because from view point of cost, it is not practical to remove liquid crystal the liquid crystal display device, correct the defective pixels, and then fill the liquid crystal in the liquid crystal display device. Thus, a technology for inspecting pixels and separating them as non-defective pixels and defective pixels before filling them with liquid crystal is important because the production cost can be reduced and defect information can be fed back to the production process in an early production stage.
A method for inspecting defective pixels of a liquid crystal display device before filling them with liquid crystal is described in a related art reference (patent document 1) disclosed as Japanese Patent Publication No. 2728748.
FIG. 1 shows a liquid crystal display device described in the related art reference (patent document 1). Reference numeral 1 represents a shift resister as a horizontal scanning circuit. Reference numeral 2 represents a gate driving circuit as a vertical scanning circuit. For simplicity, it is assumed that the liquid crystal display device has (4×4=16) pixels. Parallel output terminals of the shift resister 1 are connected to respective gates of analog switches 3a to 3d. Drains of the analog switches 3a to 3d are commonly connected to a drain of a signal switch 4. The drain of the signal switch 4 is grounded through a drain and a source of a reset switch 5. In addition, the drain of the signal switch 4 is connected to a source follower circuit 6.
Four data signal lines D1, D2, D3, and D4 are led out of sources of the analog switches 3a to 3d. Gate signal lines G1, G2, G3, and G4 are led out of outputs of the gate driving circuit 2. At each intersection of the data signal lines D1 to D4 and the gate signal lines G1 to G4, a pixel portion is disposed. Each pixel portion is composed of a pixel transistor S and a capacitor Cs. A pixel electrode (not shown) is connected to a capacitor Cs in parallel. Liquid crystal is sealed between pixel electrodes and their opposite transparent electrodes. The transparent electrodes are coated on the glass substrate.
In the normal operation of the device, pixels to which signals are sent from the shift register 1 and the gate driving circuit 2 through data signal lines and gate signal lines become active. A signal potential applied through the signal switch 4 is led to a data signal line and written to a pixel through a pixel transistor S. A capacitor Cs disposed at each pixel is an auxiliary capacitor that holds the signal potential until the next writing operation is performed.
The foregoing related art reference (patent document 1) describes a method for determining a defective pixel due to a defective transistor S, an insufficient capacitance of a capacity Cs, or the like of a pixel portion before filling it with liquid crystal. First of all, a write mode that causes a high level (sometimes denoted by “H”) voltage always to be generated through the signal switch 4 is set. In the write mode, a gate electrode, for example, G2, is set to “H” The outputs of the shift resister 1 are turned on in succession. Thus, the transistors 7 of the four pixel portions on the second row of the pixel selection are turned on in succession. As a result, signal charges are written to these pixel portions in succession.
After signal charges have been written to all the pixel portions, the gate of the signal switch 4 becomes the ground potential. The drain sides of the analog switches 3a to 3d become a high impedance state. In other words, a read mode is set. For example, the gate signal line G2 on the second row is set to “H”. Signals of all the pixel portions on the second row are read in succession. Whenever a signal of one pixel is read, the reset switch 5 is turned on. Before a signal of the next pixel portion is read, the reset operation is performed.
A signal that is read from each pixel is output through the analog switches 3a to 3d and the source follower circuit 6. An output signal of the source follower circuit 6 is observed. Corresponding to the output signal of the source follower circuit 6, pixels are inspected for defective ones. If a pixel portion at the second row and third column is defective, the source follower circuit 6 does not output a signal corresponding to the pixel portion. As a result, it can be determined that the pixel portion is defective. In other words, the technology described in the related art reference (patent document 1) is a method for detecting a waveform corresponding to a discharge amount so as to detect a defective pixel.
However, according to the method described in the related art reference (patent document 1), since defective pixels are evaluated one by one, when a high resolution liquid crystal display panel having more than 1,000,000 and 2,000,000 pixel such as (1280×1024) and (1920×1200), is evaluated for defective pixels, it takes a long measuring time to evaluate all the pixels. In addition, a system that evaluates an analog detection waveform in high accuracy would be required. Moreover, the parasitic capacitance of a data signal line is much larger (for example, 200 times larger) than the capacitance of a capacitor element disposed for each pixel. In addition, the parasitic capacitances deviate for each LCD panel. Moreover, the evaluation system, for example, a tester system has a capacitance. Since capacitances of each device and each evaluation system deviate, an obtained detected waveform deviates in, for example, the amplitude thereof. As a result, without careful consideration of a parasitic capacitance of data signal lines and a capacitance of the tester, capacitances of pixels cannot be accurately evaluated with detected values.